This invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with
switching means, inductive means, and rectifying means together forming a Buck converter connected to input terminals for connection to a supply source and output terminals for connection of the means comprising the lamp so as to supply the lamp with a current through periodic switching of the switching means by means of a control signal alternately into a conducting state during a period t.sub.on and into a non-conducting state during a period t.sub.off, the Buck converter operating in a self-oscillatory mode during stable lamp operation, and PA1 a control circuit for generating the control signal, comprising a portion for generating a switch-on signal and a portion for generating a switch-off signal. PA1 continuous mode PA1 discontinuous mode. PA1 extinguished, non-ignited lamp, PA1 breakdown in the lamp followed by a glow discharge, and transition from glow discharge to arc discharge, PA1 run-up of the lamp, PA1 stable lamp operation.
A circuit arrangement of the kind mentioned in the opening paragraph is known from European Patent Application EP-A-0 401 931 whose U.S. equivalent is U.S. Pat. No. 5,068,572. The known circuit arrangement is particularly suitable for igniting and operating a high-pressure discharge lamp which forms part of a projection TV installation.
The type of switch mode power supply called Buck converter here is also known under other designations such as downconverter, step-down converter, inductor-coupled step-down converter, direct-down converter.
Although in general the input terminals and output terminals in a Buck converter are in direct electrical connection with one another, it is equally well possible for the circuit to be provided with an electrical separation between input and output terminals, for example, in the form of a transformer.
The means comprising the lamp comprise a commutator circuit in many practical cases with the object of passing a current of alternating polarity through the lamp during lamp operation. The means comprising the lamp in general also comprise an igniter circuit for generating a voltage pulse for lamp ignition. Both the commutator circuit and the igniter circuit usually form part of the circuit arrangement according to the invention. The commutator means can be left out in the case of a lamp operating on a DC voltage.
In the known circuit arrangement, the Buck converter operates in a self-oscillatory mode which is characterized by the fact that the generation of the control signal for switching the switching means into the conducting state, i.e. the switch-on signal, is initiated the moment that the current through the inductive means has become zero, and switching takes place immediately after that.
It is possible with the known circuit arrangement to supply a substantially constant power to the connected lamp over a comparatively wide current and voltage range, so that the lamp generates a luminous flux which is constant to a high degree. Especially in the V-I range, where the lamp operates in a stable manner, the self-oscillatory mode is characterized by low switching losses in the periodic switching of the switching means. Preferably, the downconverter is dimensioned such that the frequency of the self-oscillatory mode lies above the limit of human hearing during stable lamp operation. This implies that the dimensions of the self-induction means must remain comparatively small. Switching from the conducting to the non-conducting state takes place in the known circuit arrangement when a signal proportional to the current through the inductive means becomes equal to a separately set control signal. A control of the power supplied to the lamp is possible through a control of the current at the output terminals of the circuit arrangement, for example in dependence on the voltage across the output terminals.
Although a controlled current source of comparatively simple construction is realized with the known circuit arrangement, where the power dissipated in a load (the lamp) is also controllable, the known circuit arrangement does have a number of disadvantages.
Both the period t.sub.on and the period t.sub.off are variable in the known circuit arrangement within a range which is determined only on the one hand by the maximum current through the inductive means and on the other hand by the dimensions of the inductive means and the value of the voltage at the input and output terminals (input and output voltage). Although this leads to a reliable and optimum-efficiency operation of the circuit arrangement under conditions of stable lamp operation, it leads to considerable power losses under abnormal operating conditions, which may even lead to the switching means becoming defective. There is also a risk that the switching frequency of the switching means will come to lie within the audible range.